lnwire+multi: define Error() for lnwire.Error

To make lnwire.Error actually satisfy the error interface, define the
Error method directly.

peer.go

@@ -1250,7 +1250,7 @@ func messageSummary(msg lnwire.Message) string {
 			msg.ChanID, msg.ID, msg.FailureCode)
 
 	case *lnwire.Error:
-		return fmt.Sprintf("chan_id=%v, err=%v", msg.ChanID, string(msg.Data))
+		return fmt.Sprintf("%v", msg.Error())
 
 	case *lnwire.AnnounceSignatures:
 		return fmt.Sprintf("chan_id=%v, short_chan_id=%v", msg.ChannelID,